1. Field of the Invention
This invention relates to the fabrication of complex microcircuit boards and substrates.
2. Description of the Prior Art
The art of forming shaped articles from particulate mixtures is well known. Classically a desired particulate material is mixed with a binder and then formed into the desired shape, called a green body. The green body is then fired to provide a fusion of the particulate material and drive off the binder, thereby producing the desired shape product with proper surface texture, strength, etc.
In the production of micro-electronic substrates in the manner above described, it has been a practice to mix the particulate material, usually ceramic, and a thermpolastic binder system together and cast the mixed system onto a moving belt. The cast mixture is then adjusted to a given thickness by the use of a scraper or doctor blade. The green ceramic material, thus doctor bladed to a given desired thickness, is usually permitted to evaporate out a portion of the thermpolastic binder and the green dried sheet is then cut into desired lengths for further processing.
The green ceramic sheets thus formed have had a considerable amount of effort and cost expended in maintaining them to a specific thickness and every effort is also expended to maintain the sheets throughout their processing in a configuration in which the surfaces are as flat and parallel as possible. In subsequent manufacturing steps, alternations are made to this thin, flat, parallel-faced geometry by such techniques as punching holes through the green ceramic, laser scribing the green ceramic to cause depressions therein, the passage of the green ceramic beneath a saw or grinding wheel to scribe or machine a configuration into its surface and other techniques that are very well and thoroughly described in the literature of this art.
Subsequently, additional coatings are applied to the ceramic by one of several well known techniques, such as silk screen printing on to the flat surfaces of the ceramic substrate or by filling in the grooves formed through some mechanical machining operation and either carefully removing or not applying material to those sections of the substrate between the grooves. In any case, the accepted practice starts with a ceramic substrate which is manufactured in a controlled flat configuration and is subsequently worked by a machining or other mechanical manufacturing process to alter the surface configuration to something other than a flat surface. As the green ceramic is quite delicate, the degree to which the flat, as formed, surface can be mechanically altered into a complex three dimensional configuration is severely limited and the ability to closely control the internal geometry of the grooves and holes is also severely limited.
In a complex hybrid substrate in which a great many components, both active and passive, are located on the substrate and interconnected to each other and to the outside world, the interconnection problem is extremely severe. Many components that are affixed to the substrate, such as micro-electronic circuits and transistors, are interconnected with the conductive paths formed on the substrate by means of fine wires which are welded to sensitive areas, termed pads, on the transistors or micro electronic circuits and terminated on the substrate by another weldment to a conductive area on the surface of the substrate. The wires that are utilized for these complex interconnections are extremely fine on the order of magnitude of 0.001 inches in diameter. Frequently, due to the interconnection requirements of the praticular hybrid circuit, a wire must pass over another conductive path, usually, though not necessarily limited to, a metallized region on the substrate. When this is necessary, what is usually done is to provide, through multiple silk screening and firing techniques, an insulating layer that separates the two conductive paths so that a short circuit will not occur at the region of cross over.
Another problem prevalent in conventional substrate manufacuture is that when the prepared substrate is fired, the shrinkage due to sintering is not uniform and not totally predictable. When the non-uniformity of the shrinkage of the substrate exceeds the tolerance limits of the subsequent silk screen printing operations on the surface of the substrate, it becomes impossible to register the printing on the substrate with any through holes that may exist on the substrate. This effect places a limitation on the size of the circuit being fabricated as any arbitrarily small size must be within the limits of these two independent manufacturing steps.
Another limitation found in conventional high density microcircuit boards and substrates is the dissipation of the heat generated in the high density packaged components in that the heat generated by placing components increasingly closer together becomes progressively more difficult to dissipate and remove from the surface as the circuit density increases. The conventional solution to this problem is to employ a substrate material that has the highest possible thermal conductivity. This is generally beryllium oxide which is generally considered to be highly toxic.